This invention relates generally to the development of electrostatic images, and more particularly concerns a two component development apparatus having a variable pitch auger to improve pickup latitude in developer housing.
Generally, the process of electrophotographic printing includes sensitizing a photoconductive surface by charging it to a substantially uniform potential. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to a desired image. The selective dissipation of the charge leaves a latent charge pattern that is developed by bringing a developer material into contact therewith. This process forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration.
Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface.
There are various known development systems for bringing toner particles to a latent image on a photoconductive surface. These are: single component, two component, and hybrid systems. Additionally the single component and hybrid systems may be either scavenging or scavengeless; two component development systems are almost always scavenging. The term scavenging or scavengeless denotes whether the development method would disturb any previously developed image already on the photoconductive surface. if any previously developed image is left undisturbed, the system is scavengeless.
Single Component Development Systems: A (scavenging) single component development system uses a donor roll for transporting charged toner to the development nip defined by the donor roll and the photoconductive surface. The toner is loaded onto the donor roll by direct contact with a toner reservoir and sometimes with the assistance of a toner loading brush or foam roll. The donor roll rotates to bring the charged toner into the development nip. Using a combination of AC and /or DC electrical biases, the toner is moved from the donor roll to the photoconductive surface. Thus, the toner is developed on the latent image recorded on the photoconductive surface.
A scavengeless single component development system is physically similar to a scavenging single component system except that it uses a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires detaching the toner from the donor roll and forming a toner powder cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image.
Two Component Development Systems: in a two component development system, a magnetic developer roll (with rotating external shell and an interior magnetic assembly which can be either stationary or rotating) attracts developer from a reservoir. The developer includes carrier and toner. As the external shell rotates and transports the developer material, the developer material is subsequently trimmed or metered to a desired uniform thickness. This layer of material is commonly referred to as a magnetic brush. Further rotation of the external shell advances the developer material into the development nip. In the development nip, the magnetic brush is brought into contact with the photoreceptor. Here, the toner is attracted from the carrier beads to the photoreceptor to develop the latent image. Further rotation of the developer roll returns the carrier beads and unused toner to the developer housing reservoir or sump.
Hybrid Development Systems: A hybrid development system is a cross between a single component development system and a two component system. A Hybrid system uses two component developer materials in conjunction with a magnetic developer roll to form a magnetic brush. However instead of developing the image directly with the magnetic brush, the magnetic brush is used to apply a uniform layer of toner onto a donor roll. Then as the donor roll rotates, the toner layer is advanced into the development nip and the latent image is developed in a manner similar to single component systems. A Hybrid System may be either scavenging or scavengeless.
Two component systems, either strictly two component or hybrid, require a uniform layer of developer material on the developer roll to function optimally. This layer of material must be provided independent of many factors. In some developer housing designs, developer material is picked up from one auger, trimmed to the desired thickness, used to develop an image or to load a donor roll, and then released into different auger. This results in a gradient in the developer material mass (or volume fill) down the length of the pick up auger region; one end of the auger is nearly full and the other end would be almost empty. One solution known in the prior art to deal with this variation, is to vary the “pick up” magnetic pole strength along the developer roll with a weaker pick up pole strength being used to acquire material in the almost full end of the auger and a very strong magnetic pole strength being used to acquire material from the almost empty end of the auger. An undesirable feature of this approach is that it is difficult to manufacture a magnetic structure with the appropriately varying magnetic strength.
A second solution known in the prior art is to simply use a uniform and very strong pickup magnet. An undesirable outcome of this solution is that much more material than necessary would be picked up from the nearly full end of the donor roll. This causes a small non-uniformity in the layer thickness, increases mechanical power requirements needed to rotate the donor roll, increases developer material abuse, and leads to a higher unit manufacturing cost (UMC).